PROJECT SUMMARY This project addresses the controversy on cardiomyogenesis of the adult mouse heart, and raises the unexplored possibility that the aging myopathy is dependent on alterations of cardiac stem cell (CSC) growth and differentiation. The major hypothesis to be tested is that myocardial aging is a stem cell disease and defects in CSC behavior determine the structural and physiological manifestations of the old heart. Cardiomyogenesis, mediated by CSC activation and lineage specification, may change dramatically during the lifespan of the mouse heart, and the phenotypic properties of aged CSCs may impact on the size, composition, and function of the derived progeny. Aged CSCs may form more fibroblasts and less myocytes than young CSCs, resulting in an impairment of myocyte relaxation and ventricular compliance, typical aspects of diastolic dysfunction and the cardiac senescent phenotype. However, a pool of young CSCs may persist in the old heart and could be implemented to rescue and, eventually, reverse the aging myopathy. To establish the cellular mechanisms involved in the maturation of the heart, tissue homeostasis in the adult organ, and myocardial aging, we will employ three distinct approaches, which are dependent on separate parameters: a) The average age of myocytes and non-myocytes will be determined by 14C thymidine birth dating of cardiac cells; b) The age distribution of myocytes and non-myocytes will be defined by a novel strategy based on age-structured cell populations; and c) The rate of formation of myocytes and nonmyocytes derived from commitment of CSCs will be evaluated by a model of hierarchically organized cells. These 3 sets of information will offer a redundant perspective of the cellular dynamics of the mouse heart.